Today, high capacity communication by wireless transmissions is a common phenomenon. The development and deployment of wireless cellular network have been particularly successful. A cellular network is a radio network made up of a number of radio cells each served by a fixed transceiver, known as a cell site or base station. As is well known, the cells in a cellular network are used to cover different areas in order to provide radio coverage over a wider area than the area of one single cell. A common example of a cellular network is the cell phone networks, wherein signals are communicated by means of radio waves between a mobile telephone or a similar portable communication device and a cell site (base station) or a similar access point.
There are a number of different cellular network technologies, including but not limited to: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Enhanced Data rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS) being standardized by the Third Generation Partnership Project (3GPP) including but not limited to 3GPP LTE (Long Term Evolution), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Worldwide Interoperability for Microwave Access (WiMAX), Digital Enhanced Cordless
Telecommunications (DECT), Digital AMPS (IS-136/TDMA), Integrated Digital Enhanced Network (iDEN) and similar. The invention described herein is applicable mutatis mutandis to substantially all cellular network technologies mentioned above and their similar and/or equivalent counterparts.
In cellular networks strong interferers may cause general problems for base stations or similar access points. This is particularly so if the interferer appears very close to the access point. A strong interferer may then block the base station, even if the interferer operates on another frequency channel near or adjacent to the channel selected and/or wanted by the access point in question.
A strong interferer as mentioned above may e.g. be present very close to the base station when a so called “femto base” is used. A femto base is a small cellular base station arrangement with a short range designed to be used by cell phones in a residential area or in a small business environment. Typically the short range femto base is connected to the core network of a cellular network via a communication network. In turn the cellular network comprises ordinary base stations with a long range. The range of at least one ordinary base station may cover the range of the femto base. The communication network connecting the femto base to the cellular network may e.g. be the Internet to which the femto base may be connected via a Broadband connection or similar, e.g. a Digital Subscriber Line (DSL) providing 24 Mbit/s provided via the wires of a local telephone network. A typical femto base arrangement incorporates the functionality of an ordinary base station but extended to allow a simpler, self contained deployment. For example, a UMTS femto base may contain a Node B, a Radio Network Controller (RNC) and a Gateway Support Node (GSN) with Ethernet for backhaul. When a cell phone leaves the residential area or the small business environment a handover is conducted to an ordinary base station with a longer range.
Some technical aspects of femto bases and similar access points have e.g. been studied and reported by the 3GPP in the UMTS specification TR 25.820 V8.000 (2008-03), with the title “3G Home Node B Study Item Technical Report”. Although much attention is focussed on UMTS, the concept is applicable to substantially all cellular technologies including those mentioned above and similar.
To illustrate the severity of a strong interferer near the base station we assume that a small base station such as a Home Node B or similar is used in a residential area as a part of a first cellular network provided by a first network operator. A first User Equipment (UE) being connected to the first network can then perform handover from an ordinary Node B to the Home Node B when the first UE is sufficiently close to the Home Node B. Once in the residential area the first UE will typically operate within a few meters from and often in line of sight to the Home Node B. The UE and the Home Node B may even be located in the same room. Hence, due to the short distance the Home Node B will instruct the first UE to reduce its output power accordingly.
Now, assume that a second UE being connected to a second cellular network provided by a second network operator enters the room wherein the first UE and the Home Node B are located. The second UE cannot perform handover to the Home Node B since the second UE is connected to another network. Instead, the second UE will maintain or seek connection with an ordinary Node B in the second network, which may be located hundred of metres or even kilometres from the second UE. Hence, the second UE will transmit with a much higher power than the first UE. If we assume that the first network and the second network operate under the same cellular technology (e.g. UMTS) then the frequency band used by the first network and the frequency band used second network may be quite close. This implies that sidebands of the powerful transmissions from the second UE will fall within the frequency band of the Home Node B, which will experience a strong interfering signal. Moreover, since the Home Node B is a part of the first network and the second UE is a part of the second network it follows that the Home Node B has no means for instructing the second UE to lower its output power.
To increase the signal to interferer ratio and thus to reduce the influence from the interferer one might consider to use the well known space diversity at the receiver end. However, the use of space diversity presupposes different air paths caused by reflections etc. When the interferer is very close to the base station the interferer will most likely be in line of sight to the base station, i.e. any reflections are negligible compared to the one strong path. Hence, space diversity will work very poorly or not at all with respect to such interferers.
In view of the above it would be beneficial to provide a method and a device for a simple and efficient reduction or elimination of a strong interferer situated very close to a base station or similar access point in a cellular network or similar, preferably when the interferer operates under the same radio network technology as the access point.